The invention concerns the purification of C.sub.4 and/or C.sub.5 hydrocarbon cuts containing, as impurities, water and dimethyl ether. It concerns more particularly the purification of C.sub.4 and/or C.sub.5 olefinic cuts, previously freed of at least a portion of the isobutene and/or isopentene thereof by reaction with methanol.
By isopentene, is meant mainly 2-methyl-1-butene and 2-methyl-2-butene.
It is already known to manufacture methyl tert-butyl ether (MTBE) and/or tert-amyl methyl ether (TAME) by reaction of methanol with isobutene and/or iospentene contained in an olefinic C.sub.4 and/or C.sub.5 hydrocarbon cut optionally containing other hydrocarbons. The isobutene and isopentene selectively react, whereas the other monoolefins remain practically unchanged. After separation of the methyl tert-butyl ether (MTBE) and/or tert-amyl methyl ether (TAME), the residual olefinic cut may be used as a charge in an oligomerization, particularly a dimerization, co-dimerization and/or trimerization process, as for example the so-called "Dimersol" process.
In this known oligomerization process, the olefin charge is contacted with a catalyst formed from a nickel compound, for example a nickel carboxylate, and a hydrocarbyl aluminum halide.
It has been observed that the olefinic cuts freed of isobutene and/or isopentene by reaction with methanol and separated from the one or more formed mixed ether(s) did not give satisfactory results in oligomerization. As a matter of fact, the following drawback is observed: destruction of the metal organic compounds such as hydrocarbyl aluminum halides and, consequently, blocking of the olefin oligomerization reaction.
Systematic experimentation has shown that these drawbacks are limited or even may disappear when the water and dimethyl ether content of the olefinic cut is reduced, dimethyl ether and water being by-products of the above-mentioned reaction of manufacture of MTBE and/or TAME.
It is particularly advantageous to reduce this content below 10 ppm by weight for water and below 50 ppm by weight for dimethyl ether.
Unfortunately, this removal is particularly difficult to achieve in an efficient manner without resulting in a substantial loss of olefins.
The invention provides for a process whereby the purification of said cuts may be achieved under particularly advantageous conditions.